Process for the manufacture of hydrogen fluoride

ABSTRACT

Hydrogen fluoride is produced continuously by reacting calcium fluoride (fluorspar) and sulfuric acid, at temperatures between about 250* and 600*F. in the presence of more than 3 parts calcium sulfate per part of by-product calcium sulfate produced from said reaction.

United States Patent 1191 Watson et al. [4 1 Feb. 27, 1973 [54] PROCESSFOR THE MANUFACTURE [56] References Cited F HYDROGEN FLUORIDE 0 UNITEDSTATES PATENTS [75] Inventors: William E. Watson, Mount Tabor; 2932 s574/1960 t l 423/485 18 e a g l Tmege" Chatham 3,063,815 11/1962 Redniss...423/485 x o 3,207,579 9/1965 Burkhardt 423/485 2 ud h I 3,278,26510/1966 Quarles ..423/485 [73] Asslgnee i g corpora New 3,300,279 1/1967Parisot ..423/485 [22] Filed: Sept. 20, 1971 Primary ExaminerEdwardStern Attorne -Gerard P. Roone 21 Appl. No.: 182,261 y y Related US.Application Data {57] ABSTRACT [63] Continuation of Ser. No. 876 552Nov. 13 I969, Hydrogen flumide is Produced cominuously by reactabandoned ing calcium fluoride (fluorspar) and sulfuric acid, attemperatures between about 250 and 600F. in the presence of more than 3parts calcium sulfate per part of by-product calcium sulfate producedfrom said I [58] Field of Search ..423/485,555,554, 155

4 Claims, N0 Drawings ticularly adapted to large scale production wheregood thermal conductivity and a minimum of down time due to corrosion isessential.

In the past, the reaction has usually been carried out commercially inexternally heated furnaces characterized by a metal shell provided withrails, scrapers, knockers, or other mechanical means to remove theby-product incrustations which would otherwise form on the furnacewalls, thus limiting the rate of heat transfer through the furnace wallsinto the reaction mass.

Typically, commercial processes for preparing hydrofluoric acid employfurnaces which fall into four general types and may be described asfollows:

a. One type of furnace comprises fixed horizontal metal cylinders,externally fired, and provided with rotating scrapers which serve toscrape the cylindrical walls and advance the reaction mass from the feedend to the discharge end of the furnace. In practice, this type furnacehas limited utility because the corrosion of the scraper soon increasesthe clearance between scraper and furnace wall, so that a hardincrustation is invariably present which markedly reduces heat transfer.Furthermore, the resistance offered by the incrustation to the scrapercauses very high torque on the scraper assembly, leading to highmaintenance costs and down time.

b. Another type of furnace comprises rotating cylinders, externallyheated, horizontal or nearly so, provided with loose rails or heavy barsinside, which by rotation of the furnace, scrape and tumble against thefurnace walls to remove incrustation. The constant exposure of freshmetal surfaces on the rails and furnace wall accelerates corrosion sothat furnaces of this type require frequent replacement of the rails andfurnace shells after operating for about 2000 hours or less.

c. A third type of furnace includes the use of heavy duty premixerswhich premix the calcium fluoride and sulfuric acid, either or both ofwhich are usually preheated to attain partial reaction in the mixer, toinsure that the feed to the furnace will not incrust the furnace wall.This arrangement does protect the furnace and eliminates the need forscraping devices, but the most corrosive part of the reaction is therebytransferred to the costly premixer.

d. The fourth type of furnace is the one used to form a portion of therequired sulfuric acid by reacting steam and sulfur trioxide within thefurnace. Sulfuric acid in vapor phase (S0 will not react with fluorspar,but by showering the reaction mass through the sulfuric vapor,condensation and reaction are attained. While this system eliminates theproblem of heat transfer through the furnace wall, it is relativelyexpensive to operate because of the cost of the sulfur trioxide, and thenecessity for constructing the furnace shell of costly corrosionresistant alloys to resist the effect of the presence of sulfuric andhydrofluoric acid mixtures.

The above operations have certain drawbacks which seriously affect theeconomic feasibility of a commercial process the first three haveserious limitations when it is desired to design a high capacity furnaceand the fourth is limited by high operating costs.

It is the object of this invention to provide a process for continuouslymanufacturing hydrogen fluoride from calcium fluoride and sulfuric acid,said process being applicable and practical for scale-up to high outputlevels and favorable to efficient production at low unit costs.

Another object of this invention is to provide a process in a furnacewhich may be operated without the use of rails, knockers, scrapers, ortumbling devices which lead to frequent shutdowns, maintenance, andparts replacement.

A further object of this invention is to provide a process which may beoperated in a furnace constructed of relatively low cost materialswithout recourse in its design to costly corrosion resistant alloys.

A still further object of this invention is to provide a process bywhich relatively pure hydrogen fluoride may be commercially produced athigh efficiencies.

These and other objects are accomplished according to the presentinvention wherein hydrogen fluoride is produced by the reaction ofcalcium fluoride and sulfuric acid at temperatures between about 250 and600F. in the presence of more than 3 parts calcium sulfate per part ofby-product calcium sulfate produced. Preferably, the calcium sulfate isrecycled by-product calcium sulfate withdrawn at a point in the furnaceprior to the completion of the reaction in the production of hydrogenfluoride.

The essential feature of this invention involves the use of more than 3parts of calcium sulfate per part of by-product calcium sulfate producedin the reaction mixture of fluorspar and sulfuric acid in the productionof hydrofluoric acid and which is effectively achieved without any ofthe prior art difficulties. The effectiveness of this method ofoperation is surprising and unexpected since the furnace used mayoperate without any scrapers, knockers, or other means conventionallyemployed in removing incrustation from the shell. In operation, the masswithin the furnace is substantially a free-flowing solid, and the wallsare never exposed to the action of the highly corrosive liquid sulfuricacid. The HF produced is also of higher purity than that obtained fromthe types of furnaces previously discussed, because in those typefurnaces the product gases contain large amounts of sulfur dioxide andelemental sulfur as a result of the reduction of sulfuric acid bynascent hydrogen released during the attack or corrosion of the metal ofthe furnace by the acid. Elemental sulfur is particularly objectionableas it tends to deposit on heat transfer surfaces and to plug acid dripcollection lines.

In the prior art various methods have been proposed for obviating theproblems in producing HF. One such method comprises mixing calciumfluoride and sulfuric acid by vigorous mechanical kneading prior topassage through the furnaces to assure high reaction efficiency. Suchpremixing may be necessary for high efficiency when a furnace is usedwhich depends on a single passage. Any short period failure incontacting and mixing the fluorspar and acid in a single pass furnace isin fact irreinedial and losses of expensive calcium fluoride and acidmay result.

In contrast, the present process provides for the recycle of by-productcalcium sulfate so that any short-term errors in the feed ratio of sparand acid average out and do not cause excessive losses. Also, thepresent process provides a degree of efficiency which is equal to orhigher than that previously attainable only by the use of expensivepremixers which are highly vulnerable to corrosive attack, yet theequipment employed in the present process may be of conventionalconstruction in composition and design.

The present process operates with a recycle ratio of more than about 3:!up to :1 or more, preferably a recycle ratio of about 3.521 up to about8:1 with the upper limit being dictated by size of equipment rather thanoperating efficiencies. Sound commercial practice dictates the use ofhigh recycle ratios to give better stability to the system, and allowfor other liquids such as purification system acid drips, to be returnedto the reaction zone. Also, when recycle ratios of higher values areused, the excess above the minimum serves to enfold and coat the wettedportions, thereby enhancing handling characteristics.

The use of calcium sulfate as an additive to the calciumfluoride-sulfuric acid reaction mixture in the production of hydrogenfluoride is known in the art. However, the amount of calcium sulfateemployed in these processes is less than that of this invention. It hasbeen found that the effectiveness in the use of a minimum ratio ofcalcium sulfate of more than about 3.5:1 is substantially greater thanthe systems of the prior art and is based on what is believed to be theactual reaction mechanism between sulfuric acid and calcium fluoridewhich is written overall by the simple reaction:

1. CaF H 80 CaSO ZHF As theorized, the first step comprises thereaction:

2. Cal ZH SO, Ca(HSO,) ZHF wherein the calcium bisulfate is molten atthe temperatures at which the reaction is carried out. it is believedthat it is this compound that is responsible for the characteristic andobjectionable sticky plastic stage, which in past manufacturing practicehas decomposed on the hot furnace walls to form incrustations whichinterfere with proper heat transfer.

Calcium bisulfate is then believed to react with calcium fluoride toproduce calcium sulfate and hydrogen fluoride:

3. Ca(HSO Cal 2 CaSO ZHF Calcium sulfate is also believed to react withsulfuric acid to form calcium bisulfate according to the reactron:

4. CaSO H SO -Ca(HS0 It has been found that the calcium sulfate producedby reaction (3) is readily wetted by sulfuric acid and the speed ofreaction of the residue with sulfuric acid by reaction (4) is muchfaster than reaction (3). As a result the reaction mass in aconventional HF furnace passes through the characteristic sticky,plastic stage which in contact with the hot furnace walls, plasters andincrusts.

it has been observed that this troublesome plastic stage persists untilabout 70-75 percent of the fluoride values have been driven off as HF.The reaction mass is then essentially non-sticking but damp inappearance and can easily be broken and deformed hence when the reactionis -75 percent complete, the calcium sulfate and unreacted calciumfluoride in the mixture, both of which are solids, are able to absorb orcontrol the remaining calcium bisulfate in which the unreacted portionof the sulfuric acid is held in combination. In other words, when thereaction between sulfuric acid and calcium fluoride is approximately70-75 percent complete, the troublesome plastic stage due to the liquidcalcium bisulfate, now reduced to about 40 percent by weight of thereaction mass, has been passed. Since the reaction (4) between sulfuricacid and calcium sulfate to form calcium bisulfate is much faster thaneither reaction (1) between sulfuric acid and fluorspar, or reaction (3)of calcium bisulfate and fluorspar, the first and most critical reactioncomposition occurs when the sulfuric acid reacts with a portion of thecalcium sulfate to form calcium bisulfate. At this point the presence ofa minimum of 3 moles of calcium sulfate for each mole of sulfuric acidand fluorspar is required to absorb the liquid calcium bisulfate.

While the calcium sulfate may be first mixed with the sulfuric acid, itis preferred that the fluorspar and calci urn sulfate be mixed afterwhich the sulfuric acid is introduced into this mixture. This may beaccomplished in the furnace by introducing the fluorspar at or near thepoint of the recycle of calcium sulfate, so that as the furnace rotates,the fluorspar and calcium sulfate are mixed. The sulfuric acid may thenbe introduced into the furnace at a point where it falls upon themixture and the reaction proceeds as described above. The concentrationof sulfuric acid generally employed in the manufacture of hydrofluoricacid is from about to percent, preferably 99 to 100 percent H 80 and thefluorspar contains 97 percent or more calcium fluoride and is of acidgrade quality. The amounts of sulfuric acid and fluorspar employed arein approximately stoichiometric proportions, preferably in a smallstoichiometric excess. The production of hydrogen fluoride under theconditions of the present process is substantially attained in about 5minutes up to about 4 or more hours, preferably in about 10 minutes upto minutes, with the reaction proceeding faster at the higher range oftemperatures.

The equipment may be any of the conventionally employed externally firedrotary kilns provided with means for recirculating by-product calciumsulfate to the feed end of the furnace. The recirculating means may beexternal, but preferably recirculation is achieved by internal means,thereby conserving the heat of the system. Since corrosion issubstantially eliminated in the use of the process of the presentinvention, there is no need to use special alloys in the materials usedin the furnace interior wall construction.

The following specific examples illustrate the present invention, butare not considered to be limiting the scope of the invention.

EXAMPLE 1 2700 lbs. per hour of 99.5 percent sulfuric acid and 2300 lbs.per hour of acid grade fluorspar are continuously-fed into an externallyfired rotary furnace, 5.5 feet in diameter and 17 feet long, externallyheated to maintain an internal temperature of about 450F., and rotatedat 5 RPM. No calcium sulfate is present at the feed end of the reactorwhere the fluorspar and sulfuric acid are introduced. The hydrogenfluoride gas evolved as a result of the reaction is released at the rateof 1000 pounds per hour. Calcium sulfate is removed from the furnace ata rate of about 4000 lbs. per hour. Commercial hydrogen fluoride isproduced, having the following analysis:

HF 99.2% SO, 0.5% Elemental S Present Steel rails are provided in thefurnace interior and typically after less than 1400 hours of operationthe removal of the furnace from production service will be required forreplacement of rails and extensive repairs to the shell.

EXAMPLE 2 In the same type equipment, feet in diameter and 60 feet long,means are provided for recirculating byproduct calcium sulfate to thefeed end of the furnace at a rate of about 4 parts of recycle per partof byproduct calcium sulfate produced. No rails are provided in thefurnace. To the recycled calcium sulfate is added 13,300 lbs. per hourof sulfuric acid and 11,000 lbs. per hour of acid grade fluorspar. Therotary furnace is maintained at about 2 RPM and the recirculation ofby-product calcium sulfate maintained at approximately 72,000 pounds perhour.

The temperature in the furnace is maintained at about 350-450F. Theaverage retention time of the reaction mass within the furnace is about2 hours,

wherein 5000 lbs. per hour of hydrogen fluoride is produced having thefollowing analysis:

HF 995% S0, 0.2% Elemental S Absent This furnace, after 8 months ofoperation, is still on stream. There has been very little corrosiveaction and no serious incrusta-tion problems. No more than minimalrepairs are indicated.

While the above describes the preferred embodiments of our invention forpurposes of illustration, it will be understood that various changes andmodifications can be made therein without departing from the spirit andscope of the invention.

We claim:

1. A method for preventing encrustation on the interior walls of anexternally fired rotary furnace, said furnace containing no mechanicalelements such as rails or scrapers, during the production of hydrogenfluoride which consists essentially in forming in the reaction zone atthe reaction temperature maintained therein, a free-flowing, solidmixture of calcium fluoride, sulfuric acid and from 3.5 parts up toabout 10 parts of calcium sulfate per part of by-product calcium sulfateproduced, reacting said mixture at temperatures within the range ofabout 250 to 600F. to produce hydrogen fluoride and by-product calciumsulfate and removing said hydrogen fluoride and by-product calciumsulfate from said reaction zone.

he process of claim 1, wherein the calcium sulfate mixed with thereactants is recycled by-product calcium sulfate.

3. the process of claim 1, wherein the ratio of calcium sulfate mixedwith the reactants is 4 to 8:1.

4. The process of claim 2, wherein said calcium sulfate is withdrawnfrom a point in the reaction between the calcium fluoride and sulfuricacid, where more than percent completion has been attained.

* III

2. The process of claim 1, wherein the calcium sulfate mixed with thereactants is recycled by-product calcium sulfate.
 3. the process ofclaim 1, wherein the ratio of calcium sulfate mixed with the reactantsis 4 to 8:1.
 4. The process of claim 2, wherein said calcium sulfate iswithdrawn from a point in the reaction between the calcium fluoride andsulfuric acid, where more than 70 percent completion has been attained.